An A500 keyboard on a 4000? OMG!
'Keyboard reset' _KBRST is generated through Fat Gary, so possibly it's regenerated from the KBCLK signal - I've always thought this method had only been used on the A1000, but as it seems all big box machines share it.
You can try connecting the keyboard's reset line to _KBCLK with a diode, hopefully this will cause a long enough low to trigger the reset.
